The following is a brief description of some existing detection methods. It is also a summary of relevant science to aid the reader in understanding the details of the claimed invention. It should not be taken as an admission that any of the cited art is prior art to the claims. The cited art is hereby incorporated herein by reference so that the general procedures and methods in that art that are of use to practice of the present invention need not be rewritten herein. In particular, applicant incorporates those sections related to general methods of “binding-pair” methodology, and methods for measurement of light scattering herein.
Sensitive Analyte Assays
Binding-pair (also known as ligand-receptor, molecular recognition binding and the like) techniques play an important role in many applications of biomedical analysis and are gaining importance in the fields of environmental science, veterinary medicine, pharmaceutical research, food and water quality control and the like. For the detection of analytes at low concentrations (less than about 1 picomole analyte/sample volume analyzed) fluorescent, luminescent, chemiluminescent, or electrochemiluminescent labels and detection methods are often used.
For the detection of low concentrations of analytes in the field of diagnostics, the methods of chemiluminescence and electrochemiluminescence are gaining wide-spread use. These methods of chemiluminescence and electro-chemiluminescence provides a means to detect low concentrations of analytes by amplifying the number of luminescent molecules or photon generating events many-fold, the resulting “signal amplification” then allowing for detection of low concentration analytes.
In addition, the method of Polymerase Chain Reaction (PCR) and other related techniques have gained wide use for amplifying the number of nucleic acid analytes in the sample. By the addition of appropriate enzymes, reagents, and temperature cycling methods, the number of nucleic acid analyte molecules are amplified such that the analyte can be detected by most known detection means. The high level of commercial activity in the development of new signal generation and detection systems, and the development of new types of test kits and instruments utilizing signal and analyte molecule amplification attests to the importance and need for sensitive detection methods.
However, the above mentioned methods of signal and analyte molecule amplification have associated limitations which makes the detection of analytes by these methods complicated, not easy to use, time consuming, and costly. Problems of interference of chemical or enzymatic reactions, contamination, complicated and multi-step procedures, limited adaptability to single step “homogeneous” (non-separation) formats, and the requirement of costly and sophisticated instrumentation are areas that those in the art are constantly trying to improve.
Thus, there is a tremendous need for easy to use, quantitative, multi-analyte, and inexpensive procedures and instruments for the detection of analytes. Such procedures, test kits, and instruments would overcome the disadvantages and limitations of the current methods of signal and analyte molecule amplification, and would be useful in research, individual point of care situations (doctor's office, emergency room, out in the field, etc.), and in high throughput testing applications.
It is the object of the present invention to provide a new means to more easily detect one or more analytes in a sample to low concentrations than was previously possible. The present invention can detect low concentrations of analytes without the need for signal or analyte molecule amplification.
The present invention provides a signal and detection system for the detection of analytes where the procedures can be simplified and the amount and types of steps and reagents reduced. The present invention provides for the quantitative detection of single or multiple analytes in a sample. The present invention also provides for substantial reductions in the number of different tests and amounts of sample material that are analyzed. Such reduction in the number of individual tests leads to reduced cost and waste production, especially medically-related waste that must be disposed of.
Light Scattering Detection Methods and Properties of Light Scattering Particles
There is a large body of information concerning the phenomenon of light scattering by particles, the use of particulate labels in diagnostic assays, and the use of light scattering methods in diagnostic assays which are now presented in the following discussion of relevant art none of which is admitted to be prior art to the pending claims. This art is provided as a background for understanding of the novelty and utility of the claimed invention.
The general study of light scattering comprises a very large field. The phenomena of light scattering has been studied intensely for about the last one hundred or so years and the applications of the knowledge of light scattering to different aspects of human endeavor are wide and varied.
The classical theory of light scattering by small, homogeneous, non light absorbing, spherical particles of a size of about 1/20or less the wavelength of the incident radiation was initially developed by Rayleigh. Later a more general phenomenological theory of light scattering by homogeneous, spherical particles of any size and composition was developed by Mie. The Mie theory applies both to light absorbing and non-absorbing particles. It has also been shown from Mie theory that the expressions of Rayleigh can easily be generalized so as to apply to particles which absorb light as long as the particles are much smaller than the wavelength of incident light. For these small diameter particles, Mie theory and the generalized Rayleigh theory give similar results. Light scattering (elastic) can be viewed from a classical or quantum mechanical point of view. An excellent quantitative description can be obtained through the classical point of view.
A historical background as well as a description of the basic theories of scattered light and other electromagnetic radiation is provided in the following references; Absorption and Scattering of Light By Small Particles (1983), C. F. Bohren, D. R. Huffman, John Wiley and Sons; The Scattering of Light and Other Electromagnetic Radiation (1969), M. Kerker, Academic Press.
Further background information of the phenomenon of light scattering can be found in the following publications.
Zsigmondy, Colloids and the Ultramicroscope—A Manual of Colloid Chemistry and Ultramicroscopy, 1914, John Wiley & Sons, Inc. is described various light scattering properties of gold particles and other types of particles.
Hunter, Foundation of Colloid Science, Vol, I, 105, 1991, describes use of optical microscopes, ultramicroscopes, and electron microscopes in observation of particles.
Shaw et al., Introduction to Colloid and Surface Chemistry, 2nd ed., 41, 1970, describe optical properties of colloids and the use of electron microscopy, and dark field microscopy e g., the ultramicroscope.
Stolz, SpringerTracts, Vol. 130, describes time resolve light scattering methodologies.
Klein and Metz, 5 Photographic Science and Engineering 5-11, 1961, describes the color of colloidal silver particles in gelatin.
Eversole and Broida, 15 Physical Review 1644-1654, 1977, describes the size and shape effects on light scattering from various metal particles such as silver, gold, and copper.
Kreibig and Zacharias, 231 Z. Physik 128-143, 1970, describe surface plasma resonances in small spherical silver and gold particles.
Bloemer et al., 37 Physical Review 8015-8021, 1988, describes the optical properties of submicrometer—sized silver needles and the use of such needles is described in Bloemer, U.S. Pat. No. 5,151,956,where a surface plasmon resonance of small particles of metal to polarize light propagating in a wave guide is described.
Wiegel., 136 Zeitschrift fur Physik, Bd., 642-653, 1954, describes the color of colloidal silver and the use of electron microscopy.
Use of Particles, Light Scattering and Other Methods for Detection of Analytes
For about the last thirty-five years, metal particles including gold and silver have been used as both contrast enhancement agents or light absorption labels in many different types of analytic and/or diagnostic applications. The great majority of these applications fall under the category of cytoimmunochemistry studies which have used gold or silver enhanced gold particles as markers to study structural aspects of cellular, subcellular, or tissue organization. In these studies, metal particles are usually detected and localized by electron microscopy, including scanning, transmission, and BEI (backscattered electron imaging). These methods take advantage of the electron dense nature of metals or the high atomic number of metals to facilitate the detection of the gold particles by virtue of the large numbers of secondary and backscattered electrons generated by the dense metal (see; Hayat, Immunogold-silver staining reference Page 1 and Chapters 1, 6, 15; and Hayat, Colloid Gold reference Chapters 1, 5, 7 and others).
There have been a few reports of the use of gold and silver enhanced gold particles in light microscopic studies. For example, in 1978 gold particles were used as an immunogold stain with detection by light microscopy. A review of the use of gold particles in light microscopy (See, Hayat, Immunogold-Silver Staining Reference Page 3) published in 1995 discusses this 1978 work and presents the following analysis:                “Geoghehan et al. (1978) were the first to use the red or pink color of colloidal gold sols for light microscopic immunogold staining using paraffin sections. In semithin resin sections red color of light scattered from gold particles as small as 14 nm was seen in cell organelles containing high concentrations of labeled antigens in the light microscope (Lucocq and Roth, 1984). Since the sensitivity of immunogold staining in light microscopy is inferior in comparison with other immunocytochemical techniques, the former did not gain general acceptance; the pinkish color of the gold deposit is difficult to visualize.”        
This paragraph is an indication of the state of understanding of the light scattering properties of gold and other metal particles for diagnostic and analytic studies. The paragraph specifically states “In semi-thin resin sections red color of light scatter from gold particles as small as 14 nm was seen in organelles containing high concentrations of labeled antigens in the light microscope.”
However, with white light illumination, the scattered light from 14 nm gold particles is predominantly green. Since the particles appear red in the light microscope this indicates that some interactions other than pure light scattering are being detected. It is probable that the red color observed in the light microscope is predominantly transmitted light and not scattered light. When the gold particles accumulate sufficiently at the target site in cells, tissue sections or some other surface the red color due to transmitted light will be seen (see also; J. Roth (1983) Immunocytochemistry 2: 217; and Dewaele et al (1983) in Techniques in Immunochemistry Vol 2 p1, Eds. Bullock and Petrusz, Academic Press).
As mentioned in the above quote, it appears that the sensitivity of immunogold staining in light microscopy was believed to be inferior to that of other methods, and the use of gold particles as markers for light microscope detection did not gain general acceptance. In the 1995 review book in Chapter 12, p198 by Gao and Gao is the following quote on the same subject.                “Colloidal gold was initially used only as a marker for electron microscopy (EM), because of its electron dense nature and secondary electron emission feature (Horisberger, 1979). Direct visualization of colloidal gold in light microscopy (LM) was limited. The size of colloidal gold is too small to be detected at the light microscope level, although using highly concentrated immunogold cells may be stained red by this reagent (Geoghegan et al., 1978; Roth, 1982; Holgate et al., 1983.”        
As mentioned in both of the above, the sensitivity of detection of colloidal gold with light microscopy was believed to be low. The method of silver enhancement of gold particles was developed to overcome this perceived drawback. The following is another quote from the 1995 review book.                “The real breakthrough for immunogold staining for light microscopy came with the introduction of silver-enhancement of colloidal gold particles (20 nm) bound to immunoglobin in paraffin sections 5 microns (Holgate et al., 1983). This approach significantly enhanced the sensitivity, efficiency, and accuracy of antigen detectability in the light microscope. Using IGSS, gold particles as small as 1 nm in diameter can be visualized in the light microscope. Thin section subjected to IGSS can also be viewed with the light microscope, especially by using phase contrast or epi-polarization illumination (Stierhof et al., 1992).”        
The method of silver enhancement of gold particles is widely used. The enhancement method transforms the marker gold particle into a larger metal particle or an even larger structure which is microns or greater in dimensions. These structures are composed primarily of silver, and such enlarged particles can be more readily detected visually in the bright field optical microscope. Individual enlarged particles have been visualized by high resolution laser confocal and epipolarization light microscopy. Id. at 26 and 203.
However, even with the use of silver enhancement techniques, those in the art indicate that this will not achieve the sensitivity and specificity of other methods. For example, in the publication of Vener, T. I. et. al., Analytical Biochemistry 198: 308-311 (1991) the authors discuss a new method of sensitive analyte detection called Latex Hybridization Assay (LHA). In the method they use large polymer particles of 1.8 microns in diameter that are filled with many highly fluorescent dye molecules as the analyte tracer, detecting the bound analytes by the fluorescent signal. The following excerpt is from this publication:                “To assess the merits of LHA we have compared our technique with two other indirect non-radioactive techniques described in the literature. The most appropriate technique for comparison is the streptavidin colloid gold method with silver enhancement of a hybridization signal, since this is a competing corpuscular technique. However, this method is not very sensitive even with the additional step of silver enhancement: 8pg of λ-phage DNA is detected by this method as compared to 0.6 pg or 2×104 molecules of λ DNA detected by LHA on the nylon membrane.”        
Stimpson et al., Proc. Natl. Acad. Sci. USA, 92: 6379-6383, July 1995, a real time detection method for detection of DNA hybridization is described. The authors describe use of a particulate label on a target DNA which acts as a                “light-scattering source when illuminated by the evanescent wave of the wave guide and only the label bound to the surface generates a signal. . . . The evanescent wave created by the wave guide is used to scatter light from a particulate label adsorbed at multiple DNA capture zones placed on the wave guide surface. Since an evanescent wave only extends a few hundred nanometers from the wave guide surface, the unbound/dissociated label does not scatter light and a wash step is not required. The signal intensity is sufficient to allow measurement of the surface binding and desorption of the light-scattering label can be studied in real time; i.e., detection is not rate limiting. The hybridization pattern on the chip can be evaluated visually or acquired for quantitative analysis by using a standard CCD camera with an 8-bit video frame grabber in 1/30 of a second.”        
Experiments were performed with 70 nanometer diameter gold particles and 200 nanometer diameter selenium particles. More intense signals were observed with the selenium particles. The authors indicate                “A wave guide signal sufficient for single-base discrimination has been generated between 4 and 40 nm DNA and is, therefore, comparable to a fluorescence signal system.”        
This method uses waveguides and evanescent type illumination. In addition, the method is about as sensitive as current fluorescence-based detection systems. Particles of 70 nm diameter and larger are said to be preferred. This is also described in Stimpson et al., U.S. Pat. No. 5,599,668.
Schutt et al., U.S. Pat. No. 5,017,009, describes an immunoassay system for detection of ligands or ligand binding partners in a heterogenous format. The system is based upon detection of                “back scattered light from an evanescent wave disturbed by the presence of a colloidal gold label brought to the interface by an immunological reaction. . . . Placement of the detector at a back angle above the critical angle insures a superior signal-to-noise ratio.”        
The authors explain that the immunoassay system described utilizes scattered total internal reflectance, i.e., propagation of evanescent waves. They indicate that the presence of colloidal gold disrupts propagation of the evanescent wave resulting in scattered light, which may be detected by a photomultiplier or other light sensor to provide a responsive signal. They indicate that an important aspect of their invention is the physical location of the detector.                “The detector is ideally placed at an angle greater than the critical angle and in a location whereby only light scattered backward toward the light source is detected. This location thereby ideally avoids the detection of superior scattered light within the bulk liquid medium.”        
Total internal reflection of the incident beam is used to create the evanescent wave mode of illumination and the detection is performed on an optically-transmissive surface. The use of specialized apparatus is preferred.
Leuvering, U.S. Pat. No. 4,313,734, describes a method for detection of specific binding proteins by use of a labeled component obtained by coupling particles “of an aqueous dispersion of a metal, metal compound, or polymer nuclei coated with a metal or metal compound having a diameter of at least 5 nm.” The process is said to be especially suited for estimation of immunochemical components such as haptens, antigens and antibodies. The metal particles are said to have already been used as contrast-enhancing labels in electron microscopy but their use in immunoassays had apparently                “not previously been reported and has surprisingly proved to be possible.        The metal sol particle, immunochemical technique, according to the instant invention which has been developed can be not only more sensitive than the known radio- and enzyme-immunotechniques, but renders it furthermore possible to demonstrate and to determine more than one immunological component in the same test medium simultaneously by utilizing sol particles of different chemical compositions as labels.”        
Examples of metals include platinum, gold, silver, and copper or their salts.                “The measurement of the physical properties and/or the concentration of the metal and/or the formed metal containing agglomerate in a certain phase of the reaction mixture may take place using numerous techniques, which are in themselves known. As examples of these techniques there may be cited the calorimetric determination, in which use is made of the intense colour of some dispersions which furthermore change colour with physicochemical changes; the visual method, which is often already applicable to qualitative determinations in view of the above-noted fact that metal sols are coloured; the use of flame emission spectrophotometry or another plasma-emission spectrophotometric method which renders simultaneous determination possible, and the highly sensitive method of flame-less atomic absorption spectrophotometry.”        
Two or more analytes in a sample are preferably detected by using flame emission spectrophotometry or another plasma-emission spectrophotometric method. The preferred method of detection for greatest sensitivity is by flame-less atomic absorption spectrophotometry.
Swope et al., U.S. Pat. No. 5,350,697 describes apparatus to measure scattered light by having the light source located to direct light at less than the critical angle toward the sample. The detector is located to detect scattered light outside the envelope of the critical angle.
Craig et al., U.S. Pat. No. 4,480,042 describes use of high refractive index particle reagents in light scattering immunoassays. The preferred particles are composed of polymer materials. The concentration of compounds of biological interest was determined by measuring the change in turbidity caused by particle agglutination or inhibition of agglutination. The preferred particles are of a diameter less than approximately 0.1 μ and greater than 0.03 μ. “Shorter wavelengths, such as 340 nm, give larger signal differences than longer wavelengths, such as 400 nm.”
Cohen et al., U.S. Pat. No. 4,851,329 and Hansen, U.S. Pat. No. 5,286,452, describe methods for detection of agglutinated particles by optical pulse particle size analysis or by use of an optical flow particle analyzer. These systems are said to be useful for determination of antigen or antibody concentrations. These methods use sophisticated apparatus and specialized signal processing means. Preferred particle diameters are of about 0.1 to 1 micron in diameter for the method of Cohen and about 0.5 to about 7.0 microns in diameter for the method of Hansen.
Okano et al., Analytical Biochemistry 202: 120, 1992, describes a heterogenous sandwich immunoassay utilizing microparticles which can be counted with an inverted optical microscope. The microparticles were of approximately 0.76 microns in diameter, and were carboxylated microparticles made from acrylate.
Other particle detection methods are described by Block, U.S. Pat. No. 3,975,084, Kuroda, U.S. Pat. No. 5,274,431, Ford, Jr., U.S. Pat. No. 5,305,073, Furuya, U.S. Pat. No. 5,257,087, and by Taniguchi et al., U.S. Pat. No. 5,311,275.
Geoghegan et al., Immunological Communications 7:1-12, 1978, describes use of colloidal gold to label rabbit anti-goat IgG for indirect detection of other antibodies. A light and electron microscope were used to detect labeled particles. The gold particles had an average size of 18-20 nanometers and bright field light microscopy was used. For electron microscopy, Araldite silver-gold thin sections were used. “Similar percentages of surface labeled cells were noted by immunofluorescence and the colloidal gold bright field method.” 1-5 particles per cell could be detected by electron microscopy but the authors state that:                “Such small quantities of label were not detected by fluorescence or by brightfield microscopy and may represent either non-specific and Fc receptor bound GAD and GAM, where a low level of surface immunoglobulin (S.Ig) on the GAD and GAM treated cells.”        
Hari et al., U.S. Pat. No. 5,079,172, describes use of gold particles in antibody reactions and detection of those particles using an electron microscope. 15 nanometer gold particles were exemplified. In the preferred method, electron microscopy is used.
DeMey et al., U.S. Pat. No. 4,420,558, describes the use of a bright field light microscopic method for enumerating cells labeled with gold-labeled antibodies. The method uses a light microscope in the bright field arrangement and magnifications of 500 or greater with immersion oil lenses are used to count gold-labeled peroxidase negative cells. The visualization of the labeled-surfaces is based on the aggregate properties of the gold particles, which, under the indicated circumstances, undergo extensive patching, these patches on the cell surface being resolvable with the method described. 40 nanometer gold was found to give optimal results.
De Mey et al., U.S. Pat. No. 4,446,238, describes a similar bright field light microscopic immunocytochemical method for localization of colloidal gold labeled immunoglobulins as a red colored marker in histological sections. The method of Immuno Gold Staining (IGS) as described by the authors                “In both procedures the end-product is an accumulation of large numbers of gold granules over antigen-containing areas, thus yielding the typical reddish colour of colloidal gold sols.”        
DeBrabander et al., U.S. Pat. No. 4,752,567 describes a method for detecting individual metal particles of a diameter smaller than 200 nm by use of bright field or epi-polarization microscopy and contrast enhancement with a vide camera is described. The inventors state:                “Typically, in the above mentioned procedures, the employed metal particles have a diameter of from about 10 to about 100 nm. This is well below the resolution limit of bright field microscopy, which is generally accepted to lie around 200 nm. It is therefore quite logical that all previously known visual light microscopic methods are limited in their applications to the detection of immobilized aggregates of metal particles. Individual particles could be observed with ultramicroscopic techniques only, in particular with electron microscopy.        It has now quite surprisingly been found that individual metal particles of a diameter smaller than 200 nm can be made clearly visible by means of bright field light microscopy or epi-polarization microscopy in the visible spectrum, provided that the resulting image is subjected to electronic contrast enhancement.”        
In subsequent sections the authors state:                “Compared with existing diagnostic methods based on sol particle immuno assays, the present method has a much greater sensitivity. Indeed, existing methods are in general based on light absorption or scattering by the bulk of absorbed or suspended metal particles. Obviously, the observation of colour, e.g. on a blotting medium, requires the presence of massive numbers of particles. In contrast therewith, the present method makes it possible to observe and count single particles. Hence, the present method will largely facilitate the development of diagnostic blots for applications where existing, e.g. visual or calorimetric, techniques are too less sensitive, e.g. for the detection of Hepatitis.”        
Schafer et al., Nature 352: 444-448, 1991, describes use of nanometer size particles of gold which could be observed using video enhanced differential interference contrast microscopy. A 40 nanometer diameter gold particle was used.
DeBrabander et al., Cell Motility and the Cytoskeleton 6: 105-113, 1986, (and U.S. Pat. No. 4,752,567) describe use of submicroscopic gold particles and bright field video contrast enhancement. Specifically, the cells were observed by bright field video enhanced contrast microscopy with gold particles of 5-40 nanometers diameters. They also state that                “individual gold particles, having a size smaller than plus or minus 100 nanometers, adsorbed under glass or cells or microinjected in cells are not visible in the light microscope. They are, however, easily visualized when using the capacity of a video camera to enhance contrast electronically.”        
The authors describe use of epi-illumination, with polarized light and collection of reflected light or by use of a “easier and apparently more sensitive way” with a transmitted bright field illumination using monochromatic light and a simple camera. The authors indicate that the gold particles can be easily detected with phase contrast microscopy.                “Unlike that which is possible with larger gold (usually 20-40 nm), even dense accumulations of 5-nm gold, e.g., on structures such as microtubules, are not visible in the light microscope. They do not produce a detectable red colour. Recently, this has been corrected by a physical development with silver salts, which increases the size of the particles to produce all easily visible black stain.        We have described a method for localizing ligands almost at the molecular level. The method is new because it enables one for the first time to do this in the light microscope with discrete individual markers that are unambiguously discernible from background structures. Because it is applicable even in living cells, one can thus follow the dynamic behaviour of individual proteins. The method is because it combines two well developed techniques: gold labelling and video microscopy. Most of the applications can be done with inexpensive video equipment the price of which is less than most good 100× oil objectives. Still, many more possibilities arise when combining this with modern digital image manipulations. Some additional advantages are worth noting. Because the label consists of individual discrete markers, both manual and automatic (computer assisted) counting is easy and reliable. The small size of the marker minimizes problems of penetration and diffusion. The possibility of changing the charge of the marker almost at will is helpful in diminishing nonspecific binding in any particular application.”        
This method was termed by the authors “nanoparticle video ultramicroscopy or short nanovid ultramicroscopy.” A similar technology is described in “Geerts et al., Nature 351: 765-766, 1991.
Analyte detection using light scattering particles as labels is described in Yguerabide et al. PCT/US/97/06584 (WO 97/40181 and Yguerabide et al., PCT/US98/23160 (WO 99/20789). Elements of the technology are also described in two related articles by Yguerabide & Yguerabide, (1998) Anal. Biochem. 261:157-176; and (1998) Anal. Biochem. 262:137-156.
Methods utilizing light scattering (referred to as “plasmon resonance”) labels for assays are also described in Schultz, et al, PCT/US98/02995 (WO 98/37417). The methods described in Schultz et al. are generally based on detection of individual plasmon resonant particles or entities.
Phillips et al. (U.S. Pat. No. 6,171,793 B1) describes a method of extending the dynamic range of a detector for gene probe arrays labeled with fluorescent reporter groups. The method involves the collection of light signals from multiple sites on the array at two different wavelengths, where the signal intensity at one wavelength can exceed the dynamic range of the detector for some sites of the array, while the signal intensity at the other wavelength is within the range of the detector. A scale factor correlation function is calculated based on a functional fit to a plot of the detector response at the two different wavelengths for all of the sites on the array. For sites that generated a signal which saturated the detector at one wavelength, the scale factor correlation function is applied to the signal at the other wavelength where it is within the dynamic range of the detector, such that the signal at the wavelength where the detector is saturated can be extrapolated.
The preceding discussions of the state of the art of light scattering methods, and the use of light scattering particles and methods in the field of diagnostics clearly shows the limits of current methods of analyte detection and the novelty and great utility of the present invention. It is the purpose of this invention not only to overcome the present day limitations and disadvantages of light scattering-based diagnostic assays, but to also overcome the limitations and disadvantages of other non-light scattering methods such as signal and analyte molecule amplification. This invention as described herein is easier to use, has greater detection sensitivity, and is capable of measuring analytes across wider analyte concentration ranges than was previously possible. The present invention is broadly applicable to most sample types and assay formats as a signal generation and detection system for analyte detection.